Jökull - 01.12.1979, Page 54

Fig. 4. Typical temperature profiles from deep
drillholes inside and outside geothermal areas in
Iceland. Kaldársel is within the active volcanic
zone but far away from high temperature areas and
shows nearly zero thermal gradient down to about
700 m. Holes in the Westman Islands and Akranes
show thermal gradients nearly undisturbed by
water convection. Laugaland LJ-8 and Reykjavik
G-4 are typical for production holes in low tem-
perature areas, whereas the holes Krafla and
Krísuvík are in high temperature areas. (Slightly
modified from Pálmason et al. 1978).
HIGH TEMPERATURE AREAS
According to the plate tectonics theory the
highest heat flow on a constructive plate margin
should be along the volcanic zone, which is the
surface expression of the plate boundary. This is
not always apparent on the surface as recent vol-
canics are normally highly pervious and cold
groundwater percolates deep into the surface for-
mations. In one drillhole in the volcanic zone of
SW-Iceland a zero thermal gradient was encount-
ered down to 700 m. With increasing compaction of
the strata and sealing by precipitation from warm
water the geothermal gradient increases with
depth, and it is likely that magmatic temperatures
(1000—1200°C) prevail at 10 km depth or less un-
der the entire volcanic zone of Iceland. The high
temperature areas are like chimneys that extend
from the hot zone below and all the way to the
surface. The high temperature areas are always
associated with volcanotectonic features such as
volcanic fissure swarms or more commonly central
volcanoes with intermediate and acid volcanics,
fault swarms, and sometimes calderas. At such sites
there is a great abundance of dykes, sheets and
other minor instrusions cooling at a shallow depth
in the crust. These intrusions, in addition to the
general heat flux of the volcanic zone, form the heat
source for the hot water convection systems of the
high temperature areas. The mineral content of the
water increases with its temperature. Precipitation
of mainly silica and calcium carbonate occurs
where the high temperature water meets cold
groundwater. This seals the hot water cell off and
with time allows it to extend up through the per-
vious surface formations.
Several new high temperature areas have been
identified with increasing research during the last
decade. To date there are considered to be 22 cer-
tain and 3 potential high temperature areas in the
country. The surface manifestations are in the form
of steam holes, boiling mudpools and highly altered
ground. The high temperature areas vary greatly in
size and have an aggregate coverage of about 500
km2. Three areas cover approximately 100 km2 or
more, but the bulk of the areas are 1 — 20 km2. The
size of the individual high temperature areas is a
function of the age of the systems, the extent of the
magmatic heat source and the lithology and struc-
ture of the strata in which the high temperature
convection systems are formed. The total natural
heat discharge of the high temperature areas is
poorly known, but has been estimated at about
4000 MW.
The heat exchange between the intrusives and
the meteoric water can to some extent be inspected
in the deeply dissected roots of Tertiary and Plio-
Pleistocene central volcanoes. These are charac-
terized by a great abundance (locally 50— 100%) of
minor intrusions. Centrally inclined sheet swarms
(cone sheets) have been found in the majority of
dissected central volcanoes investigated to date in
Iceland. The sheets are commonly 1 — 2 m thick.
Minor dolerite, gabbro and granophyre intrusions
are also common. The host rock is intensely altered
and the cores of the central volcanoes are charac-
terized by a cupola of propylitized rocks which
delineates fhe shape of the extinct high tempera-
ture convection system. The outer part of the
aureole is characterized by quartz and platy calcite,
but these minerals are accompanied by laumontite
52 JÖKULL 29. ÁR